STRUCTURE OF FERRIC SOYBEAN LEGHEMOGLOBIN-A NICOTINATE AT 2.3-ANGSTROM RESOLUTION

Soybean leghemoglobin a is a small (16 kDa) protein facilitating the t ransport of O-2 to respiring N-2-fixing bacteria at low free-O-2 tensi on. The crystal structure of soybean ferric leghemoglobin a nicotinate has been refined at 2.3 Angstrom resolution. The final R factor is 15 .8% for 6877 reflections between 6.0 and 2.3 Angstrom. The structure o f soybean leghemoglobin a (143 residues) is closely similar to that of lupin leghemoglobin II (153 residues), the proteins having 82 identic al residues when the sequences are aligned. The new structure provides support for the conclusion that the unique properties of leghemoglobi n arise principally from a heme pocket considerably larger and more fl exible than that of myoglobin, a strongly ruffled heme group, and a pr oximal histidine orientation more favourable to ligand binding.